Fire proof oriented strand board and its manufacture

ABSTRACT

The invention pertains to a method of manufacturing a fire proof board from strands of wood, the method comprising the following steps in the given order: (a) providing strands of wood, (b) immersing the strands of wood in a watery solution of a flame retardant chemical composition, (c) separating the strands of wood from each other, (d) at least in part drying the strands of wood, (e) applying an adhesive to the strands of wood, and (f) forming a board of a predetermined shape from the strands. The invention furthermore pertains to a board formed by a plurality of strands of wood that are compressed and bonded together with an adhesive, wherein the board further comprises a flame retardant chemical composition, said composition comprising a phosphate compound.

TECHNICAL FIELD

The present invention pertains to a method of manufacturing a fire proofboard from strands of wood, and to a ire proof board formed by strandsof wood.

BACKGROUND OF THE INVENTION

Oriented strand board, also known as OSB, waferboard, Sterling board orExterior board and SmartPly is a widely used engineered wood productformed by strands (flakes) of wood, often layered in specificorientations. In appearance, it may have a rough and variegated surfacewith the individual strands (typically around 2.5 cm by 15 cm each)lying unevenly across each other. OSB's are cheep and strong boards, andthis makes them excellent building material.

Most countries have certain fire prevention regulations setting certainstandards for the fire retardancy of building materials for indoor use,especially for use in public buildings. However, it is a problem withflame retardant agents used in the industry today that they are toxic tohumans and animals. Building material treated with these known toxicframe retardant agents will inevitably release some of it into thesurrounding environment, which makes building materials that are treatedwith such toxic agents unsuitable for indoor use.

In WO 03/099533 is disclosed a method of providing flame retardant OSBplates. According to the disclosed method, wood pieces are impregnatedwith a flame retardant composition before they are processed intostrands. The impregnation with the flame retardant is done by theapplication of a so called vacuum-pressure process, wherein the woodfirst is subjected to a vacuum; subsequently the flame retardantcomposition is added and subjected to pressure in order to impregnatethe wood pieces. These impregnated wood pieces are then processed intostrands that are subsequently used to produce OSB plates in atraditional way.

In WO 01/53821 is disclosed another method of producing flame retardantOSB plates. According to the disclosed method, the strands are ledthrough a sprinkling device, wherein they are sprayed with a watersolution of a flame retardant mineral composition comprising ammoniumsulfate and/or ammonium phosphate. Next to the sprinkling device an airheater is placed. The strands are then carried with this hot air througha dryer. From these impregnated strands fire retardant OSB plate areproduced.

In WO 97/46635 is disclosed a fire retardant composition for use in OSBplates, said composition comprising a mixture of Ammonium sulfate, Boraxand trinatrium phosphate. Furthermore is disclosed an OSB platecomprising such a fire retardant composition, and a method of providinga fire retardant OSB plate. The method involve the steps of impregnatingthe strands with the above mentioned mixture in a watery solution, andsubsequently drying them to a humidity of between 1% to 12%. After thisdrying, the strands are coated with an adhesive and formed into plates.

However, In order to produce fire retardant OSB plates in large scaleproduction facilities in an error free and effective manner, severaltechnical problems, which are not addressed by any of the abovementioned documents, need to be carefully considered. In order toprovide an efficient method of producing fire retardant OSB plates, itis necessary to perform the impregnation of the strands at a pace, whichwill not interfere with, or halt, the running of the rest of the processsteps associated with the large scale production of OSB plates.Furthermore, some of the fire retardant chemical composition may seepout of the strands and cover its surface, so that the subsequentlyapplied adhesive will not bind the strands properly together, whichagain leads to a useless and week OSB plate.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide an improvedmethod of manufacturing fire proof boards formed by strands of wood,which may be applied in large scale production of said boards, whereinthe above mentioned and other problems are alleviated.

It is a further objective of the present invention to provide afireproof board formed by strands of wood, which is suitable for indooruse as a building material.

According to the present invention, the above-mentioned and otherobjects are fulfilled by a method of manufacturing a fire proof boardfrom strands of wood, the method comprising the following steps in thegiven order.

(a) providing strands of wood,(b) immersing the strands of wood in a watery solution of a flameretardant chemical composition,(c) separating the strands of wood from each other,(d) at least in part drying the strands of wood,(e) applying an adhesive to the strands of wood, and(f) forming a board of a predetermined shape from the strands.

By immersing the strands in a watery solution of a flame retardantchemical composition, virtually each individual strand of wood in theboard is made flame retardant. However, in order to achieve an efficientand timesaving impregnation of the strands with the flame retardantchemical composition, the strands are separated from each other inprocess step (c). Investigations have shown that the impregnation timecan be reduced from approximately 12 hours to some few minutes if thisseparation of the strands is property achieved. The method steps (b)-(d)can be implemented in already existing machinery for the production offor example oriented strand boards, wherein method steps (a), (e) and(f) may be exercised in a manner known in the art of producing boards ofstrands of wood. For example typically in step (a), the strands are cutfrom a log of wood. When immersed in the watery solution of the flameretardant chemical composition, the strands thus absorb this waterysolution until they are saturated. In the step (d), the strands are atleast in part dried, whereby the water is at least in part removed fromsaid strands, but the fire retardant chemical composition remains withinthe strands. Hereby the strands are impregnated by the fire retardantchemical composition before the adhesive is applied to the strands andthey are formed into a board of a predetermined shape.

Advantageously, the step (c) of separating the strands from each otheris performed substantially simultaneously to the step (b) of immersingthe strands in a watery solution of a flame retardant chemicalcomposition, i.e. the strands are advantageously separated from eachother in the watery solution of a flame retardant chemical composition.Hereby is achieved that virtually all the surface of each individualstrand is exposed to the watery solution of a flame retardant chemicalcomposition, thereby increasing the efficiency of the impregnation ofthe strands.

In an embodiment, the step (c) of separating the strands from each othermay comprise stirring the mixture of strands and watery solution of aflame retardant chemical composition. This may for example be done bymechanical stirring means know in the art. Investigations have howeversurprisingly shown that it increases the impregnation efficiency (e.g.reduce the time needed to impregnate the strands) if this stirring isachieved by sending air bobbles through the watery solution of a flameretardant chemical composition.

Preferably, the step (a) of providing the strands of wood comprises thestep of providing strands of wood having a humidity between 5% and 25%,preferably between 5% and 20%, even more preferably between 8% and 20%.Investigations have shown that for moist commonly used wood species(e.g. aspen or poplar) the impregnation is most efficient if the strandsare provided with a humidity between 12% and 17%, which hence is themost preferred range of humidity of the strands that are used in thepresent method.

In a preferred embodiment, the method further comprises a step ofmeasuring whether the provided strands of wood have the desired humiditybetween 5% and 25%, preferably between 56% and 20%, even more preferablybetween 8% and 20%, or even more preferably between 12% and 17%, priorto the step (b).

Preferably, the method further comprises the step of increasing thehumidity of the strands if the measurement shows that they have ahumidity which is lower than the desired one, or pre-drying the strandsif the measurement shows that they have a humidity that is larger thanthe desired one. Hereby it is achieved that the strands have the optimalhumidity when they are immersed in the watery solution of a flameretardant chemical composition, i.e. that they have a humidity whichfacilitates the most efficient (quickly) impregnation of the strands.This is of critical importance because, if it is not possible toimpregnate the strands quickly enough, then the impregnation of thestrands will negatively affect the other further processing steps usedin manufacturing OSB plates.

When the strands are removed from the watery solution of a flameretardant chemical composition, they are so wet that the adhesives,which are usually used in the manufacturing of OSB plates, will not bondto them. Therefore, the step (d) of drying the strands of wood comprisesthe step of drying the strands of wood to a humidity of between 4% and10%, preferably between 4% and 8%, such as for example about 6%.

Preferably, the step (d) of drying the strands of wood comprises thestep of drying the strands of wood in such a way so that substantiallynone, or negligibly little, of the fire retardant chemical compositionseeps out of the strands and deposits on the surface of said strands.

In a preferred embodiment, the step (d) of drying the strands of woodcomprises the step of drying the strands for 2 minutes-10 minutes,preferably for 2 minutes-8 minutes, even more preferably for 2 minutes-6minutes, yet even more preferably for 3 minutes-4 minutes.Investigations have shown that if the strands are dried too fast, or ata too high temperature, then parts of the chemical composition seeps outof the strands again and deposits on the surface of them. If thishappens, the adhesives applied in OSB manufacturing will not be able tobond properly to the strands, why it is impossible to use the strands inthe production of OSB plates. Investigations have shown that if thestrands are dried for the periods mentioned above, then it is possibleto preclude the problems associated with seeping out of parts of thefire retardant chemical, composition, and it is possible to produce OSBplates from them which do not fail standardized stress tests. However,in order to implement the step (d) in large scale production of OSBplates, the slowness of the drying must be balanced against the desiredspeed of the overall process, which is reflected in the preferredintervals mentioned above.

In a preferred embodiment, the watery solution of the fire retardantchemical composition comprises an unsaturated solution of the fireretardant chemical composition. Hereby the problems associated withimpregnating too much of the fire retardant chemical composition areavoided. These problems are for example that the structure of the woodenstrands may be weakened or at least in part destroyed, thereby weakeningthe strands. This causes the OSB plates produced from such strands to betoo week.

Preferably, the watery solution of the fire retardant chemicalcomposition comprises less than 25% by weight of said solution,preferably between 15% and 22% by weight of said solution, such as 18.5%by weight of said solution. Tests performed by the applicant havesurprisingly shown that optimal results are achieved if between 15% and22% by weight of a ire retardant chemical composition is used.

In an embodiment, the step (b) of immersing the strands of wood in awatery solution of a flame retardant chemical composition comprises thesub step of immersing the strands of wood in a watery solution of aflame retardant chemical composition for less than 10 minutes onaverage, preferably between 2 minutes and 6 minutes, even morepreferably between 3 minutes and 4 minutes on average. Tests performedby the applicant have surprisingly shown that if the strands areseparated, e.g. by stirring, then the strands will be sufficientlyimpregnated with the fire retardant chemical composition if they areimmersed in said watery solution for less than 10 minutes on average,preferably between 2 minutes and 5 minutes, even more preferably between3 minutes and 4 minutes on average. Hereby it will be possible tobalance the need for keeping up with the general production speed of anOSB plate manufacturing plant and sufficient impregnation of the strandswith the fire retardant chemical composition.

In a preferred embodiment, resin adhesives are used alone or incombination with wax. For example 95% by weight of strands of wood and6% by weight of wax and resin.

In a preferred embodiment the step (f) of the inventive method furthercomprise the sub steps of:

-   -   aligning the strands so that they are substantially parallel to        each other,    -   compressing the aligned strands and adhesive with pressure into        a predetermined shape, and    -   curing said compressed mixture of strands and adhesive to        produce a board of strands having a predetermined shape.

Alternatively, the step (f) further comprises the sub steps of

-   -   forming at least two layers of strands, wherein the strands of        each layer are substantially parallel to each other,    -   placing the layers on top of each other in such away that the        strands of two abutting layers are not parallel to each other,    -   compressing the layers of strands and adhesive with pressure        into a predetermined shape, and    -   curing said compressed mixture of strands and adhesive to        produce a layered board of strands having a predetermined shape.

The layers may be created by shredding wood into strands, which aresifted and then oriented on a belt or wire cauls. Then a met is made ina forming line, where the layers are built up with the external layersaligned in the strength axis of the board with internal layerscross-oriented. The number of layers placed is determined partly by thethickness of the panel, but is limited by the equipment installed at themanufacturing site. However, individual layers can also vary inthickness to give different finished panel thicknesses (typically, a 15cm layer will produce a 15 mm board thickness).

In yet an alternative embodiment, the step (f) further comprises the substeps of

-   -   compressing the strands end adhesive with pressure into a        predetermined shape, and    -   curing said compressed mixture of strands and adhesive to        produce a board of strands having a predetermined shape.

Hereby a simple way of producing the boards is achieved, wherein noaligning is required.

In a preferred embodiment, 60 to 70 percent of the strands are 5 to 20cm long, while the remaining 30 to 40 percent of the strands arenormally smaller than 5 cm and act as fillers when compressed to formthe board.

The mat is placed in a thermal press to compress the strands and bondthem by heat activation and curing of the resin that has been coated onthe strands. Individual boards may then be cut from the mats intofinished sizes

In one embodiment, the adhesive is a sulfonated phenol-formaldehyderesin with a curing catalyst. In one embodiment, the compressing of thestrands and adhesive with pressure into a predetermined shape isperformed at a pressure of about 4200 kPa, and cured at a temperature of160 degrees Celsius.

In a preferred embodiment, the step (d) of drying the strands of woodcomprises the step of subjecting the strands of wood to an air-jet,which has a direction that is substantially opposite to the force ofgravity on the strands. Hereby the air-Jet will remove the moisture andwater content from the strands, whereby the strands become increasinglylighter. The pressure from the air-jet could be balanced in such a wayagainst the force of gravity on the strands that when the water contentof the strands is sufficiently low, the strands are moved further up ina pipe or tunnel until they reach a level wherein the water content issufficiently low for them to be moved further to the stage, wherein theadhesive is applied to it. The process could in these stages be entirelyairborne.

In one embodiment, the step (e) of applying an adhesive to the strandsof wood comprises the sub step of spraying the adhesive on the strandsof wood and/or placing the strands of wood in the adhesive.

In a preferred embodiment, the flame retardant chemical compositioncomprises a phosphate compound or a phosphorous material. The phosphatecould for example be ammonium phosphate (CAS no. 10124-31-9), althoughother types of phosphates, such as potassium or sodium phosphate, can beused. Thus, it is contemplated that any type of phosphate compound canbe used. A preferred embodiment of the invention includes ammoniumphosphate dibasic (CAS no. 7783-28-0). However, it is contemplated thatother forms of ammonium phosphate can be used, such as monobasicammonium phosphate (CAS no. 7-722-76-1), ammonium polyphosphate, or someother type of ammonium phosphate compound. The chemical structure ofammonium phosphate dibasic makes it an especially suitable ingredientfor some embodiments of the invention.

In a preferred embodiment, the flame retardant chemical compositioncomprises a pH regulating compound. Although the pH can vary throughouta wide range, the pH of the composition is preferably maintained withinthe pH range of about 5 to 9, more preferably about 6.5 to 7.8. In someembodiments the pH is about 7.

Preferably, the pH regulating compound is a weak organic acid. By a weakacid is meant an acid that dissociates incompletely, i.e. It does notrelease all of its hydrogens in a solution, donating only a partialamount of fts protons to the solution. These acids have higher pKa thanstrong acids, which releases all of their hydrogen atoms when dissolvedin water. By an organic acid is meant an organic compound with acidicproperties. For example the organic adds may be the carboxylic acids,whose acidity is associated with their carboxyl group —COOH or forexample Sulfonic acids, containing the group —SO₂OH, which arerelatively stranger acids. Generally the relative stability of theconjugate base of the acid determines its acidity. Other groups can alsoconfer acidity, usually weakly: —OH, —SH, the enol group, and the phenolgroup. For example any of the following acids could be used: Lacticacid, Acetic acid, Formic acid, Citric acid, Oxalic acid, Uric acid.

The pH can also be adjusted through the use of appropriate phosphatesalts, or by addition of small amounts of strong adds, such as HCL, orstrong bases, such as NaOH. Citric acid may be an especially suitablematerial, as it is a relatively gentle acid that may appeal to thebuying public, government regulators, or any other persons who havereason to review the ingredients of the invention. The amount of acidicmaterial will depend on the type of the acidic material used and canvary throughout a wide range.

In some embodiments of the invention, a preservative material is addedto prevent growth of bacteria or mold during transport and/or storage ofthe composition. Any suitable preservative can be used to serve thispurpose. Preferably, the preservative material provides a source ofbenzoate ion. In some embodiments, the source of benzoate ion is abenzoic acid, and sodium benzoate is the preferred choice.Natriumbenzoate may be especially suitable, as it is a naturallyoccurring preservative that may appeal to the buying public, governmentregulators, or any other persons who have reason to review theingredients of the invention. In an alternative embodiment, Bronopol(CAS no. 52-51-7) can be used as the preservative material.

Preferably, the preservative, material is added in sufficient amount toinhibit the growth of bacteria and mold in the composition for a desiredperiod of time. In embodiment in which sodium benzoate is used, theamount can vary throughout a wide range. It is contemplated that theranges may shift depending an the type of preservative material used.Advantageously, sodium benzoate in a range between about 0.9% and 2%eliminates all bacteria and mold for at least one year for a compositionthat is kept at room temperature and in a dosed container.

Furthermore, the source of benzoate ion works as an accelerator for thefire retardant composition, partly because it effectively alters thesurface tension of the watery solution of the fire retardant chemicalcomposition in such a way that it is able to spread evenly on andthroughout the material (in this case strands of wood) to which it isapplied.

In a preferred embodiment, the phosphate compound forms between 5% and30% by weight of the flame retardant chemical composition. Although awide range of concentration of ammonium phosphate can be included,ammonium phosphate dibasic preferably makes up about 2% to about 30% ofthe composition when an aqueous solvent is also present in thecomposition. More preferably, dibasic ammonium phosphate makes up about9% to about 23% of the composition, and most preferably, it makes upabout 14% to 18% of the composition. It is contemplated that theseranges may shift depending on the type of phosphate containing materialthat is used.

In one embodiment, the pH regulating compound forms between 0.25% and10% by weight of the flame retardant chemical composition. Preferably,citric acid is used as the pH regulating compound, and it preferablymakes up about 0.25% to about 10% or 0.25% to 4% by weight of thecomposition when an aqueous solvent is also present in the composition.More preferably, the citric acid makes up about 0.75% to about 2% byweight of the composition, and most preferably, it makes up about 0.9%to 1.1% by weight of the composition. It is contemplated that theseranges may shift depending on the type of acidic material used.

In another embodiment the preservative compound forms between 0.25% and15% by weight of the flame retardant chemical composition. Inembodiments in which sodium benzoate is used as the preservativecompound, the amount can vary throughout a wide range, but preferablythe sodium benzoate makes up about 0.25% to about 15% or 0.25% to about7% by weight of the composition when an aqueous solvent is also presentin the composition. More preferably, sodium benzoate makes up about0.75% to about 4% by weight of the composition and most preferably, itmakes up about 0.9% to 2% by weight of the composition. It iscontemplated that these ranges may shift depending on the type ofpreservative material that is used. Advantageously, sodium benzoate in arange between about 0.9% and 2% by weight eliminates all bacteria andmold for at least one year for a composition that is kept at roomtemperature and in a closed container.

In one preferred embodiment the flame retardant chemical compoundcomprises in a mixture: ammonium phosphate, a source of citrate ion, asource of benzoate ion, wherein there is 1 part by weight of the sourceof citrate ion, 12.7 to 20 parts by weight of ammonium phosphate, and0.8 to 2.2 parts by weight of the source of benzoate ion.

Tests have shown that the composition is especially effective when theingredients are combined within these ranges of ratios.

The ingredients of the flame retardant composition can be combined inany suitable way to make the composition. An example of one manner bywhich the ingredients can be combined for a composition that includes anaqueous solvent will now be discussed. Approximately 50% of the aqueoussolvent can be poured in to a pan or other container of appropriatesize. Ammonium phosphate dibasic can be added while stirring rapidly,and stirring can continue for about 10-15 minutes until the ammoniumphosphate dibasic is completely dissolved. Citric acid can then be addedwhile continuing to stir rapidly for about 5 minutes. Sodium benzoatecan then be added while continuing to stir rapidly for about another 5minutes until the liquid is clear. The remaining 50% of aqueous solventcan then be added while continuing to stir the solution forapproximately 5 additional minutes. Ideally, the composition will beclear, and the ingredients will be completely dissolved Without anyvisible traces of solid material. This is only one example as to how theingredients can be combined, and it will be apparent to a person skilledin the art that there exist numerous other mixing methods that can beused.

An example of one manner by which the ingredients can be combined for acomposition that, at least initially, does not include an aqueoussolvent will now be discussed. Ammonium phosphate dibasic can be pouredinto any type of conventional powder mixer and mixed until all lumps wedissolved. Citric add can then be added and mixed in the powder mixeruntil all lumps are dissolved and the ingredients are thoroughly mixedtogether. The sodium benzoate dibasic can be added next and mixed in thepowder mixer until all lumps are dissolved and all of the ingredientsare thoroughly mixed together. The composition may now be sold or storedin this granular or powder form. At any desired time, the granular orpowder mixture can be dissolved in an aqueous solvent. The solution canbe mixed until the granular powder mixture is dissolved, contains nolumps or visible traces of solid material, and the solution is clear.

The composition can be used in various types of conditions, but someconditions can make the composition especially effective. For example,applying the composition to dry materials in temperatures at 10 degreesCelsius or above has proven to be effective. However, the composition iseffective when applied under alternative conditions. In one embodiment,the composition can be applied by spraying it onto a dry material orsubmerging a dry materiel into the composition. Preferably, the materialis allowed to dry after being saturated with the composition. This onlyprovides one example of how the composition can be used and should notbe interpreted as a limitation to the invention.

An object of the invention is furthermore achieved by a board formed bya plurality of strands of wood that are compressed and bonded togetherwith an adhesive, wherein the board further comprises a flame retardantchemical composition, said composition comprising a phosphate compound.

Hereby is achieved a flame retardant board which does not have thedetrimental environmental health-related side effects as boards treatedwith the flame retardant compositions known in the art. Since phosphateis generally non-hazardous to the health and life of humans and othermammals, possible emissions of residue phosphate into the surroundingenvironment of the board do generally not lead to any impairment of thehealth or life of mammals. Thus, a fireproof board is provided that maybe suitable for indoor use as building material in residential houses,office buildings or other buildings for humans and/or animals.

In an embodiment of the board, it is a major part of the individualstrands of the board that comprises the flame retardant chemicalcomposition in an embodiment of the board, the phosphate compound is anammonium phosphate.

In an embodiment of the board, the flame retardant chemical compositionfurther comprises a pH regulating compound. In an embodiment of theboard, the pH regulating compound is a week organic add.

In an embodiment of the board, the flame retardant chemical compositionfurther comprises a preservative compound. In an embodiment of theboard, the preservative compound is a source of benzoate ion.

In an embodiment of the board, the phosphate compound forms between 5%and 30% by weight of the flame retardant chemical composition.

In an embodiment of the board, the pH regulating compound forms between0.25% and 10% by weight of the flame retardant chemical composition.

In an embodiment of the board, the preservative compound forms between0.25% and 15% by weight of the flame retardant chemical composition.

An object of the invention is also obtained by a board formed by aplurality of strands of wood that are compressed and bonded togetherwith an adhesive, wherein the board further comprises a flame retardantchemical composition, wherein the flame retardant chemical compoundcomprises in a mixture: ammonium phosphate, a source of citrate ion, asource of benzoate ion, wherein there is 1 part by weight of the sourceof citrate ion, 12.7 to 20 parts by weight of ammonium phosphate, and0.8 to 2.2 parts by weight of the source of benzoate ion.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings. In the following, preferred embodimentsof the invention are explained in more detail with reference to thedrawings, wherein

FIG. 1 shows how a preferred embodiment of how the inventive method maybe exercised,

FIG. 2 shows an embodiment of a board according to the invention, and

FIG. 3 shows a flow diagram of an embodiment of a method according tothe invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the invention are shown. The invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete and will fully conveythe scope of the invention to those skilled in the art Like referencenumerals refer to like elements throughout. Like elements will thus notbe described in detail with respect to the description of each figure.

FIG. 1 schematically shows how an embodiment of the inventive method maybe exercised. First, logs 2 of wood are placed on a conveyer 4, whichmoves the logs 2 to a knife drum flaker 6. The knife drum flaker 8slices the logs 2 so that elongated wood flakes are produced havingtheir fiber elements parallel to the length of the flake. The flakes ofwood particles produced by the knife drum flaker 6 may be stored in astorage bin 8 until needed or conveyed directly to a hammer mill 10,which further splits the flakes into smaller wood strands of a desiredaverage width and length, thereby providing strands of wood.

The wood particles are separated into three (or optionally more)fractions, two of which are used to form the layers of a multilayerboard. The third fraction contains oversized particles that areunsuitable for use in forming the layers and therefore are refinedfurther to form usable particles. In the arrangement of the equipmentillustrated in FIG. 1, the wood particles are first passed through aparticle separator 14. The particle separator 14 is designed to classifyand separate the wood particles into three fractions; fines 21,acceptable strands (referred to herein as “strands” 23), and oversizedparticles.

In order to ensure that the strands 23 of wood have the desired humiditybetween 5% and 25%, preferably between 5% and 20%, even more preferablybetween 8% and 20%, or even more preferably between 12% and 17%, theyare led through a measurement stage 3, wherein a plurality of moisturesensors 5 detect the humidity of the passing strands 23 of wood. Themoisture sensors could in one embodiment be capacitive sensors.

If the measurement shows that the humidity of the strands 23 is lowerthan the desired humidity, the strands are led to a processing stage 7,wherein a plurality of sprinklers 9 or atomizers are used to increasethe humidity of the strands 23. In one embodiment the quantity of waterthat is applied to the strands 23 through the plurality of sprinklers 9or atomizers is dosed in dependence of the measured humidity.

On the other hand, if the measurement shows that the strands 23 have ahumidity that is larger than the desired one, then the strands 23 areled to a processing stage, wherein a pre-drying takes place in apre-dryer 11. In one embodiment, the pre-drying includes a step of usinga heated air-jet, and in one embodiment the pre-drying is performed independence of the measurement of the humidity of the strands 23.

Hereby it is achieved that the strands 23 have the optimal humiditybefore being immersed into the watery solution of the fire retardantchemical composition.

Then the fines 21 and strands 23 of wood are immersed in a waterysolution of a flame retardant chemical composition 12, wherein the fines21 and the strands 23 of wood are soaked. The watery solution of a flameretardant chemical composition 12 is placed in a container 16, which isstirred by releasing air bobbies 18 from the bottom of said container16. This stirring causes the strands 23 to be separated from each otherthereby exposing substantially all their surfaces to the watery solutionof a fire retardant composition 12, which again leads to a veryefficient and time saving impregnation. The air bubbles can be pumpedinto the container 16 using an air pump 17. In alternative embodimentsother kind of stirring means, e.g. mechanical stirring means, could beused. When the strands of wood have been immersed in the watery solutionof a fire retardant composition 12 for a sufficiently long time, such asfor example less than 10 minutes, e.g. between 2 minutes and 6 minutes,such as for example 3 minutes or 4 minutes, they are removed from thewatery solution of a fire retardant composition 12, for example by usinga screw conveyor or other means (not illustrated).

Ater the fines 21 and strand 23 of wood have been removed from thewatery solution of a fire retardant composition 12, they are too wet tobe formed into boards. Thus they are subsequently passed through anadditional drying apparatus 20 and 22 wherein they are at least in pertdried, i.e. dried to a desired moisture level. In the drying apparatus22, the strands 23 are subjected to an air-jet illustrated by the arrow24. This air-jet 24 has a direction that is substantially opposite tothe force of gravity on the strands 23. Hereby the air-jet 24 willremove the moisture and water content from the strands 23, whereby thestrands 23 become increasingly lighter. The pressure from the air-jet 24is balanced in such a way against the force of gravity on the strands 23that when the water content of the strands 23 is sufficiently low, theyare moved further up in a pipe or tunnel 26 until they reach a levelwherein the water content is sufficiently low for them to be movedfurther to the stage wherein the adhesive is applied. The air-jet 24 maybe generated by a blower 25. The dryer 22 works analogously to the dryer20 and will therefore not be described in further detail.

The fines 21 and strands 23 fractions are then stored in theirrespective bins 30 and 28 until needed for forming the board. A cyclone(not shown) may be used to dispel the air flow and discharge the strandsinto the bins 28 and 30, which also function as a buffer. In alternativeembodiments, the discharge bins 28 and 30 are optional.

As wood particle fines 21 and strands 23 are needed to form the boards,they are transferred from their respective bins 30 and 28 to meteringbins 34 and 32 operatively associated with blenders 38 and 36. Eachmetering bin 32, 34 serves to discharge wood particles into itsoperatively associated blender 36, 38 at a controllably constant feedingrate. Each blender 36, 38 is also arranged to receive an adhesive (andother additives, if desired) at a controlled feeding rate. The blenders38, 38 thoroughly mix the received components to form adhesive/woodparticle admixtures. The component feeding rates are controlled toproduce the adhesive-to-wood particle ratios desired for the admixtures.Adhesive is dispensed to each of the blenders 38 and 38 by aconventional resin mixer (not illustrated). The resin mixer serves tomix the individual components forming the adhesive, and other additiveswhen used, and meter the required quantity of adhesive to the blender 38and/or 38. Any of the resins and associated components used to makeadhesives for forming common wood particleboards can be used to make theadhesive for forming the wood particleboard of the present invention.While different adhesive-to-wood particle ratios may be used in the fineand strand admixtures (usually, a lower adhesive-to-wood particle ratiois preferred for admixtures of fines), mixers are available that cansimultaneously dispense adhesive at different feeding fates. In anyevent, a variety of adhesives may be used to form the fine and strandadmixtures, and the admixtures can be formed to have any of a wide rangeof adhesive-to-wood particle ratios. For example, urea formaldehyde,phenol formaldehyde, melamine modified urea or di-isocyanate resin basedadhesives, or combinations of them, can be used to form the admixtures.The type of adhesive and the adhesive-to-wood particle ratio selectedfor the fine and strand admixtures is usually determined by the intendeduse of the end product, the properties desired for the end product, andthe process selected to form the board. Phenol formaldehyde resin basedadhesives can be used in the production of multilayer particleboardsintended for most end-product uses, including about 8% of such adhesivein the fine admixture and about 6% of such adhesive in the strandadmixture produces satisfactory bonding. Preferably, the overallmoisture content of the strand admixture is within the range of about 8%to 13%, while that of the fine admixture is within the range of about 8%to 20%, on an oven dry weight basis. If it is not convenient or possibleto use a single resin mixer to furnish the adhesives for both the fineand strand admixtures, separate resin mixers can be arranged to dispensethe desired adhesive separately to the separate blenders 36 and 38 thatproduce the fine and strand admixtures, respectively.

The blended admixtures of adhesive coated fines 21 and adhesive coatedstrands 23 are conveyed to feeding bins associated with the woodcompression device 40 employed to form the oriented strand boards 42.The blended admixtures of adhesive coated fines 21 and adhesive coatedstrands 23 are conveyed to the compression device 40, wherein the fines21 and strands 23 are mixed in a desired ratio and compressed into apredetermined shape, and cured in order to produce a board 42 of strands23 and fines 21 having a predetermined shape. Alternatively, onlystrands 23 could be used in producing the board 42.

In the compression device 40, strands 23 (and possible also fines 21)are subjected to the necessary pressure and temperature conditions toeffect compressing and curing of the adhesive, whereby an integral,highly compacted wood particleboard 42 body is produced.

A wide range of pressure and temperature combinations can be employed toform the particleboard. The press pressure and temperature conditionscan be manipulated to either enhance Certain properties or to obtain thebest overall combination of properties in the produced particleboard.The particular temperature and pressure required to produce a desiredset of properties in a multilayer particleboard of particularconstruction can be determined empirically. Temperatures up to 230degrees Celsius and pressures in the range of 35 to 50 kilopounds persquare centimeter (kp/sqcm) can be used to form a particleboard.Following completion of the press cycle, which requires 1 to 10 minutes,depending upon the thickness and desired density of the finishedparticleboard, they are placed on a roller conveyor 44 for delivery tostorage or further processing equipment. The additional processingequipment will typically include edge and end trim saws for formingstraight edges and ends in the particleboard sections and cross-cut andrip saws for cutting the sections into desired pro-cut lumber sizes. Inaddition, the formed particleboard sections can be cut to obtain lumberhaving a single surface layer. This is accomplished by cutting theformed particleboard generally along its length-width plane. Forexample, by sawing the formed particleboard at a slight angle to itslength-width plane, beveled siding lumber with a single surface layercan be produced. Hence, It will be appreciated that a wide variety oflumber can be produced from Such a particleboard.

While a particular arrangement of equipment has been described thus farfor producing a particular particleboard, other arrangements ofequipment will also produce that and other embodiments of a board. Forexample, pre-compression press apparatus and pre-heating radio frequency(RF) heating apparatus can be included in the production line. The useof pre-compression and pre-heating apparatus reduces the overallpressing time required to form the boards, especially it multilayerparticleboards are produced. Also, mat weighing apparatus and mat rejectapparatus can be included in the particleboard production line in orderto remove from the production line improperly formed mat sectionsusually, detectable as an incorrect mat section weight. The removed orrejected mat sections can, for example, be conveyed by a swivel conveyorto a met destruction device and the resulting material returned to thewet flake storage bin 8.

The method can also include machinery for aligning the strands so thatthey are substantially parallel to each other and/or forming at leasttwo layers of strands, wherein the strands of each layer aresubstantially parallel to each other, and machinery for placing thelayers on top of each other in such a way that the strands of twoabutting layers are not parallel to each other, thereby producing anoriented strand board with crossing layers of strands.

FIG. 2 shows an embodiment of an oriented strand board 42 according tothe invention, which board comprises three layers 46, 48, 50 of strands23, two identical surface layers 46 and 50, and one intermediate layer48. Furthermore, the embodiment illustrated is a long rectangular boardconfiguration commonly used for finish-grade lumber. However, as shouldbe readily apparent from the entire description of the presentinvention, the illustrated number of layers and configuration of themultilayer wood particleboard is merely exemplary and can be variedwithout departing from the scope of the present invention.

The plurality of strands 23 of wood are compressed and bonded togetherwith an adhesive (for example as described above). The board 42 furthercomprises a flame 36 retardant chemical composition, said compositioncomprising a phosphate compound.

As illustrated in FIG. 2, each layer 46 is composed substantially ofthin wood strands 23, which have a length several times greater thantheir width and which are produced so that their fiber elements aresubstantially parallel to their length. While the wood strands 23 aregenerally straight, they have an irregular contour. The Strands 23 aredistributed in each layer 46, 48, 50 relative to each other with theirlength in substantial parallel orientation and extending in a directionapproaching a line which is parallel to one edge of the layer 46, 48,60. The wood strands 23 have a length in the range of about 12 mm to 150mm, a width in the range of about 2.0 mm to 20.0 mm and a thickness inthe range of about 0.2 mm to 0.6 mm. The wood strand layer 46, 48, 50can include wood particles having dimensions outside the above-specifiedranges and usually include a significant amount of wood particle finesas fill for voids that would otherwise exist in the layer 46, 48, 50because of the irregular contour of the strands 23. The layers 48, 48,50 can include up to about 20%, on an oven dry weight basis, woodparticle fines without degrading the strength, durability and stabilityproperties of the board 42.

With respect to the parallel orientation of the strands 23, FIG. 2 showsstrands 23 crossing one another in a woven fabric-like pattern whilehaving a mean direction parallel to one edge of the layer 46, 48, 50. Inaccordance with the present invention, parallel orientation is used todefine a distribution of wood strands 23 in which the average acuteangle between crossing strands is less than about 40 degrees and themean direction of the stands is parallel to one edge of the layer 46,48, 50.

The three layers 46, 48, 60 are arranged in a layer-to-layercross-oriented strand pattern. In other words, the top 46 and bottom 50outer layers are formed to have their strands 23 parallel and thecentral layer 48 is formed to have its strands 23 perpendicular to thoseforming the outer layers 46 and 50. The multilayer board 42 can beformed of any number of two or more layers 46, 48, 50. However, superiorstrength, durability and stability properties are achieved by amultilayer construction of three or a greater odd number of layers oforiented wood strands arranged in a layer-to-layer arose-oriented strandpattern. In addition, improved bending and tensile strengths areobtained by orienting the strands 23 in a multilayer board composed ofan odd number of layers so that the strands 23 in the outer two layers46, 50 have their lengths extending in the direction of the lengthdimension of the board 42 and the strands 23 of inner layer(s) arecross-oriented to form the desired layer-to-layer cross-oriented strandpattern.

In an alternative embodiment, both the top 50 and bottom 48 layers arecovered by a surface layer (not illustrated), composed substantially ofwood particle fines distributed in a particular unoriented graduatedsize pattern. The fines are composed of irregularly contoured woodparticles having width and thickness considerably less than the averagewidth of the wood strands 23 forming the three layers 46, 48, 50. Suchwood particle fines may be in the form of short elongated particle,pellet shaped particles and/or long, thin wood fiber particle. Forconstructing multilayer boards having core layers formed by wood strands23 having average dimensions in the above-specified ranges, fines havinga width and thickness less than an upper limit of about 0.50 mm are usedto form the two surface layers (not shown). The surface layers caninclude up to about 30%, on an oven dry weight basis, oversized woodparticles having widths and/or thicknesses greater than theabove-specified upper limit without impairing the ability to form asmooth surface finish, particularly, if the oversized wood particles areconfined to a region of the surface layers at or near the adjacent outerlayers 46 end 50, respectively.

FIG. 3 shows a flow diagram of an embodiment of a method according tothe invention, said method comprising the following steps in the givenorder:

(a) Providing strands of wood, illustrated by the box 52.(b) Immersing the strands of wood in a watery solution of a flameretardant chemical composition, Illustrated, by the box 54.(a) Separating the strands of Wood from each other, illustrated by thebox 58.(d) At least in part drying the strands of wood, illustrated by the box58.(e) Applying an adhesive to the strands of wood, illustrated by the box60.(f) Forming a board of a predetermined shape from the strands,illustrated by the box 62.

The flame retardant chemical composition mentioned with reference to anyof the FIG. 1, 2 or 3 is preferably a flame retardant chemicalcomposition (in a watery solution) as described in the section “summaryof the invention”, the description of which will therefore not berepeated here.

LIST OF REFERENCE NUMBERS

In the following is given a list of reference numbers that are used inthe detailed description of the invention.

-   2 log-   3 measurement stage for measuring humidity-   4 conveyor-   5 moisture sensors-   6 knife drum flaker-   7 processing stage for moisturizing the strands-   8 storage bin-   9 sprinklers or atomizers-   10 hammer mill-   11 pr-dryer for pre-drying the strands-   12 watery solution of a fire retardant composition-   14 particle separator-   16 container for watery solution of a fire retardant composition 12-   17 air pump-   18 air bobbles-   20,22 drying device-   21 fines of wood-   23 strands of wood-   24 air jet-   25 blower-   26 tunnel or pipe-   28, 30 storage bins-   32, 34 metering bins-   36, 38 blenders-   40 compression device-   42 board, oriented strand board, particleboard-   44 conveyor-   46 bottom layer of a board-   48 central or intermediate layer of a board-   50 top layer of a board-   50-62 method steps

1. A method of manufacturing a fire proof board from strands of wood,the method comprising the following steps in the given order: (a)providing strands of wood, and then measuring whether the providedstrands of wood have the desired humidity of between 5% and 25%,preferably between 5% and 20%, even more preferably between 8% and 20%,and then increasing the humidity of the strands if the measurement showsthat they have a humidity that is lower than the desired one, orpre-drying the strands, if the measurement shows that they have ahumidity that is larger than the desired one, (b) immersing the strandsof wood in a watery solution of a flame retardant chemical composition,(c) separating the strands of wood from each other, (d) at least in partdrying the strands of wood, (e) applying an adhesive to the strands ofwood, and (f) forming a board of a predetermined shape from the strands.2. The method according to claim 1, wherein the step (c) of separatingthe strands from each other is performed substantially simultaneously tothe step (b) of immersing the strands in a watery solution of a flameretardant chemical composition,
 3. The method according to claim 2,wherein the step (c) of separating the strands from each other comprisesstirring the mixture of strands and watery solution of a flame retardantchemical composition.
 4. The method according to claim 1, wherein thestep (d) of drying the strands of wood comprises the step of drying thestrands of wood to a humidity of between 4% and 10%, preferably between4% and 8%, such as for example about 6%.
 5. The method according toclaim 1, wherein the step (d) of drying the strands of wood comprisesthe step of drying the strands for 2 minutes-10 minutes, preferably for2 minutes-8 minutes, even more preferably for 2 minutes-6 minutes, yeteven more preferably for 3 minutes-4 minutes.
 6. The method according toclaim 1, wherein the watery solution of the fire retardant chemicalcomposition comprises an unsaturated solution of said chemicalcomposition.
 7. The method according to claim 1, wherein the waterysolution of the fire retardant chemical composition comprises less than25% by weight of said solution, preferably between 15% and 22%, such as18.5% by weight of said solution.
 8. The method according to claim 1,wherein the step (b) of immersing the strands of wood in a waterysolution of a flame retardant chemical composition comprises the substep of immersing the strands of wood in a watery solution of a flameretardant chemical composition for less than 10 minutes on average,preferably between 2 minutes and 6 minutes, even more preferably between3 minutes and 4 minutes.
 9. The method according to claim 1, wherein thestep (f) further comprises the sub steps of aligning the strands so thatthey are substantially parallel to each other, compressing the alignedstrands and adhesive with pressure into a predetermined shape, andcuring said compressed mixture of strands and adhesive to produce aboard of strands having a predetermined shape.
 10. The method accordingto claim 1, wherein the step (f) further comprises the sub steps offorming at least two layers of strands, wherein the strands of eachlayer are substantially parallel to each other, placing the layers ontop of each other in such a way that the strands of two abutting layersare not parallel to each other, compressing the layers of strands andadhesive with pressure into a predetermined shape, and curing saidcompressed mixture of strands and adhesive to produce a layered board ofstrands having a predetermined shape.
 11. The method according to claim1, wherein the step (f) further comprises the sub steps of compressingthe strands and adhesive with pressure into a predetermined shape, andcuring said compressed mixture of strands and adhesive to produce aboard of strands having a predetermined shape.
 12. The method accordingto claim 1, wherein the step (d) of drying the strands of wood comprisesthe step of subjecting the strands of wood to an air-jet, which has adirection that is substantially opposite to the force of gravity on thestrands.
 13. The method according to claim 1, wherein the step (e) ofapplying an adhesive to the strands of wood, comprises the sub step ofspraying the adhesive on the strands of wood and/or placing the strandsof wood in the adhesive.
 14. The method according to claim 1, whereinthe flame retardant chemical composition comprises a phosphate compound.15. The method according to claim 1, wherein the flame retardantchemical composition comprises a pH regulating compound.
 16. The methodaccording to claim 1, wherein the flame retardant chemical compositioncomprises a preservative compound.
 17. The method according to claim 14,wherein the phosphate compound is an ammonium phosphate.
 18. The methodaccording to claim 15, wherein the pH regulating compound is a weakorganic acid.
 19. The method according to claim 16, wherein thepreservative compound is a source of benzoate ion.
 20. The methodaccording to claim 14, wherein phosphate compound forms between 5% and30% by weight of the flame retardant chemical composition.
 21. Themethod according to claim 14, wherein pH regulating compound formsbetween 0.25% and 10% by weight of the flame retardant chemicalcomposition.
 22. The method according to claim 14, wherein preservativecompound forms between 0.25% and 15% by weight of the flame retardantchemical composition.
 23. The method according to claim 1, wherein theflame retardant chemical compound comprises in a mixture: ammoniumphosphate, a source of citrate ion, a source of benzoate ion, whereinthere is 1 part by weight of the source of citrate ion, 12.7 to 20 partsby weight of ammonium phosphate, and 0.8 to 2.2 parts by weight of thesource of benzoate ion. 24-38. (canceled)